Allow specifying QIODevice for QSettings datastore access

Allow specifying which QIODevice is used for QSettings. This way, QSettings can operate on e.g. a QFile subclass that can compress/encrypt on demand.

COMMENT: With the current QSettings design, this is unfortunately harder than it might seem without breaking binary compatibility. We'll see what we can do. As a work-around, you could consider using registerFormat() and perform the encryption/compression there.